We will determine whether chronic administration of dobutamine will increase cardiac reserve and promote the formation of coronary collaterals to the ischemic myocardium. Progressive coronary stenosis will be produced in dogs by an Ameroid constrictor around the left anterior descending coronary artery plus a fixed stenosis of the left circumflex coronary artery. Effects of cardiac conditioning will be manifested by decreases in resting heart rate, and in blood lactate, pyruvate and catecholamine concentrations. Cardiac reserve will be determined by left ventricular pressure function curves and by changes in cardiac output and arteriovenous oxygen difference during treadmill exercise before and after a dobutamine infusion program; exercise conditioning is characterized by a more efficient utilization of oxygen in tissues and a larger arteriovenous oxygen difference at a given workload. Left ventricular wall thickness measured by piezoelectric crystal techniques will be used to determine whether left ventricular hypertrophy occurs after chronic administration of dobutamine. The magnitude of coronary collateral formation to ischemic regions of the myocardium at rest and their functional adequacy in response to treadmill exercise will be determined by injecting radioactive microspheres before and after the dobutamine infusion program.